(a) Field of the Invention
The present invention relates to a 90.degree. phase shift circuit and, more particularly, to a 90.degree. phase circuit for generating I-signal and Q-signal having a phase difference of 90.degree. therebetween and suitable for use in a radio communication system.
(b) Description of the Related Art
90.degree. phase shift circuits are generally used in radio communication systems. Referring to FIG. 1, a conventional 90.degree. phase shift circuit includes a high-pass filter (HPF) 12 having therein a serial capacitor (C) an a parallel resistor (R), a low-pass filter 13 having therein a serial resistor (R) and a parallel capacitor (C), a first limiter amplifier 14 for receiving an output from the HPF 12 to deliver a Q-signal, a second limiter amplifier 15 for receiving an output from the LPF 13 to deliver an I-signal.
An input signal having an angular frequency of .omega. and supplied through the input terminal 16 common to HPF 12 and LPF 13 leads in phase after passing the HPF 12, and lags in phase after passing the LPF 13. The ratio of an output voltage V1 from the HPF 12 to an output voltage V2 from the LPF 13 is expressed by: EQU V1/V2=j.omega..multidot.C.multidot.R.
As understood from the above equation, the phase difference between the output voltages V1 and V2 is 90 degrees irrespective of the angular frequency .omega..
The practical phase error of the output signals from the HPF 12 and the LPF 13 depends on the relative accuracy between the capacitances (C) and between the resistors (R). In general, if the fabrication techniques used for current semiconductor integrated circuits are applied to the conventional 90.degree. phase shift circuit, a high relative accuracy can be obtained between the resistors and between the capacitors, and accordingly, the phase error can be suppressed to a suitable level.
On the other hand, the amplitudes of the output voltages V1 and V2 coincide with each other at an angular frequency .omega. wherein the following equation holds: EQU .omega..multidot.C.multidot.R=1.
Accordingly, the amplitude error depends on the absolute accuracy of the resistors and the capacitors. Thus, in the conventional 90.degree. phase shift circuit, limiter amplifiers are provided at the outputs of the HPF 12 and the LPF 13 for controlling the amplitude difference between the output voltages V1 and V2 to compensate the absolute errors of the resistors and the capacitors.
However, in the conventional 90.degree. phase shift circuit as described above, since the phases of the outputs from the limiter amplifiers 14 and 15 depend on the respective input amplitudes of the limiter amplifiers 14 and 15, a phase error is generated by the fact that the amplitude error is suppressed by using the limiter amplifiers. That is, the presence of the limiter amplifiers 14 and 15 itself generates a new phase error.